1. Field of the Invention
The present invention relates to the field of wireless local area networks (LANs) and, more particularly, to a method and apparatus for implementing a protocol for controlling transmitter power in wireless LANs.
2. Description of the Related Art
Through the merging of computer and communications technology, computer networks have greatly enhanced the computing power available to the individual computer user linked to other computers in a network. Not only do networks provide for the exchange of information between autonomous computers, but they also enable each user or "node" to share resources common to the entire network. Through resource sharing, all application programs, databases and physical equipment in the network may be made available to any node without regard to the physical location of the resource or the user.
As for the linkage between nodes, there are generally two types of network interconnections. The nodes in a wired network communicate with each other by using transmission lines to carry the signals between nodes. The nodes in a wireless network, on the other hand, communicate with each other using radio signals or other types of wireless links.
One type of wireless network is a wireless local area network (LAN). A LAN is local in the sense that the transceiver nodes are located within a radius of only a few miles of each other. As such, the proximity of the nodes permits the network to operate reliably at low power and at high data rates.
Typically most nodes are mobile and transmit information in packets. These mobile nodes can be configured around one or more stationary base stations, which coordinate the activity of the nodes in the network. Alternatively, the network can be set up in a free configuration, wherein the nodes communicate with each other without the need for any intermediary base stations to control network traffic. In other words, the number of nodes in a wireless LAN is typically variable and unknown to the individual nodes. A representative example of a wireless LAN is a network consisting of lap-top computer/transceiver units that are transported between offices and laboratories on a university campus.
Wireless LANs are to be contrasted with wide area networks (WANs). WANs operate over a much larger radius and accordingly require higher power transmitters. The greater distances also result in a higher error rate, thereby requiring that data be transmitted at a lower data rate, typically less than one million bits per second.
Another mobile communication system in common use is the cellular telephone network. Computer communications in cellular telephone systems cannot be accomplished optimally because the cellular telephone system is first and foremost a telephone system that is designed to optimally carry voice-encoded data, which has completely different characteristics than those of a general computer data interchange. Moreover, in the cellular telephone system, a telephone call ties up a dedicated bandwidth for the duration of the call, whereas a wireless LAN packet radio transmission occupies; a channel only while data is being transmitted.
Because of the characteristics of the broadcast channels used in wireless networks, wireless communication between nodes suffers from a number of problems. First, in the common LAN many users may be transmitting simultaneously, resulting in interference at the receiver that may cause the entire message to be obliterated. A special case of interference is the near-far problem, common in spread spectrum systems, wherein between two equally powered transmitters, the transmitter closer to the receiver has a much higher likelihood of being received, thus preventing the farther signal from successfully reaching the target receiver.
Another problem with mobile telecommunications concerns the power consumed by the mobile units. A large percentage of the weight of a mobile computer terminal is taken up by a battery. Accordingly, to reduce battery weight and increase battery life, it is desirable to keep the transmitter power and the accompanying battery drain at a minimum.
These two problems are highlighted in the case of the transition (or "hand off") of a mobile transmitter from one base station to another in a multiple base station network. As the mobile unit moves away from one base station towards another, it is desirable that at some point communications between the first base station and the mobile unit cease and become picked up by the second base station being approached by the transmitter. By maintaining communications with only the most proximate base station, the mobile unit can minimize power consumption and induce less interference with other nodes in the network if the unit has the capability to adjust its power.
The foregoing problems can be alleviated by providing for a way to adjust the transmission power of the nodes in the LAN. To minimize power consumption and reduce interference, each transmitter should transmit with just enough power to reach the intended receiver and be reliably received. By using the minimum amount of power to achieve this objective, the transmitter avoids unnecessary interference with unintended receivers. Moreover, such power control permits for spatial reuse of the bandwidth at closer distances than would otherwise be possible.
Although power control is desirable, it is difficult to accomplish in practice. The power setting must be performed at the transmitter while the measurements are taken at the receiver. The receiver therefore must somehow provide feedback to manipulate the transmitter to transmit at optimal power.
One possible solution provides that the transmitter adjust its power based upon the quality of a signal from the receiver measured at the transmitter. This method fails, however, because radio links are asymmetric, meaning that channel characteristics are not the same in both directions. Thus, the transmitter cannot determine whether a receiver has satisfactorily received the transmitted signal based upon how well the transmitter receives a signal from the receiver.
Another potential solution is a configuration in which the receiver measures signal quality and provides feedback to the transmitter while it is transmitting, instructing the transmitter to increase or decrease transmitted power based upon the received signal quality. This relative power adjustment method is workable in a synchronous circuit switching environment, such as a cellular telephone system, characterized by the continuous transmission of a single conversation back and forth between nodes.
However, in an asynchronous data transmission environment, such as a packet switching local area network, a particular transmitter typically transmits relatively short packets or datagrams at high data rates intermittently in a noncontinuous manner, which makes it quite difficult to adjust the power while the transmission is ongoing. Thus, the receiver will have to adjust the transmitter's power using a separate (possibly even special) message or packet. The packets communicated between these nodes occasionally get lost in transmission and need to be retransmitted. Moreover, nodes in the network sometimes erroneously determine that a packet sent by the node has been lost, when in fact it has not. Thus, if a receiver operating on the basis of relative power adjustment mistakenly determines that the feedback signal was not received by the transmitter, then it will retransmit the feedback packet. As a result, the transmitter will have been erroneously instructed twice to increase or decrease radiated power, revealing a major drawback to the relative power adjustment method in an asynchronous data transmission environment.
It is also desirable to provide a power control system that can operate in a wireless LAN with an open communication protocol; that is, a protocol that can be used in a network comprising transceiver nodes of different makes, which may have varying capabilities in terms of transmission power setting and signal quality measurement. Some nodes may or may not be able to adjust the transmission power, measure the quality of the received signal, or execute an algorithm to translate signal quality measurements into power settings.